High magnetic permeability cast alloy



United States Patent HIGH MAGNETIC PERMEABILITY CAST ALLOY Roy R.Albertzart, Jr., Saginaw, Mich., assignor to General Motors Corporation,Detroit, Mich, a corporation of Delaware No Drawing. Filed May 20, 1966,Ser. No. 551,528 US. Cl. 14835 3 Claims Int. Cl. C22c 37/00 ABSTRACT OFTHE DISCLOSURE A ferritic cast iron having a high magnetic permeabilityis disclosed. In a preferred form the composition contains 2.2-2.7%carbon, 1.22.0% silicon, 0.01-0.10% titanium and the balancesubstantially all iron, and has been subjected to a malleabilizing heattreatment so as to be substantially free of flake graphite and matrixpearlite.

This invention relates to a castable ferrous alloy having high magneticpermeability and more particularly to a cast ferrous alloy which issuitable for applications requiring a magnetic permeability in excess of1600 gausses per oersted at a magnetizing force of 5 oersteds and 210gausses per oersted at a magnetizing force of 70 oersteds.

Alternators have replaced generators in the ignition systems of mostautomobiles produced in the United States. Low-carbon steel stampingscommonly have been used as rotor poles for such alternators. However, inapplications requiring increased magnetic permeability, I have foundthat a cast iron rotor pole composition offers advantages over a steelstamping. I have also found that a castable ferrous alloy may be usedadvantageously to form other components requiring high magneticpermeability such as, for example, a magnetic clutch disk for airconditioner compressors.

The principal object of the present invention therefore is to provide acast ferrous metal alloy having a high magnetic permeability which canbe produced by conventional methods at a reasonable cost. This and otherobjects are attained with a cast iron of essentially ferriticmicrostructure comprising by weight 0.01-0.10% titanium, 2.2-2.7%carbon, -1.2-2.0% silicon and the balance substantially all iron. A castmalleable iron having the following composition has been found to beparticularly useful in applications requiring high magneticpermeability: carbon 2.5- 2.7% by weight, silicon 1.2-2.0%, titanium0.01-0.10%, manganese 0.300.60%, sulfur 0.15% maximum, phosphorus 0.05%maximum and the balance substantially all iron. In addition to thecomposition ranges specified, it is critical that the cast material havea microstructure comprised of ferrite and graphite, the graphite beingpresent in the form of nodules or temper carbon. The presence ofcombined carbon as pearlite or of uncombined carbon as flake graphite,adversely affects the magnetic permeability of my composition. I havefound that cast irons comprising by weight 2.2 to 2.7% carbon, 1.2 to2.0% silicon, 0.01 to 0.10% titanium and the balance substantially alliron, and further having a ferritic microstructure which issubstantially free of combined carbon as pearlite, or of uncombinedcarbon as flake graphite, have an unexpectedly high magneticpermeability of 1600 gausses per oersted or higher at a magnetic forceof 5 oersteds and 210 gausses per oersted or higher at a magnetic forceof 70 oersteds. This of course means that when the above specified castiron is actually subjected to a magnetic force of 5 oersteds a magneticflux density of at least 8000 gausses is obtained, or to a magneticforce of 70 oersteds a magnetic flux density of at least 14,700 gaussesis obtained.

The respective concentrations of carbon and silicon 3,433,685 PatentedMar. 18, 1969 which are suitable for use in my ferrous composition arewithin the cast iron range and methods and equipment for melting andcasting such compositions are well known. However, I have found thatwhen titanium is incorporated into a ferritic cast iron of the abovecomposition, the magnetic permeability is increased to the point atwhich the composition is useful in automobile alternators and the like.In accordance with my invention, titanium preferably is added in amountscomprising from 0.01-0.10% of the weight of the alloy as titanium metal,ferrotitanium, titanium-silicon alloy or in any other suitable combinedform which does not introduce unwanted impurities.

While carbon, silicon, and titanium are the critical alloyingconstituents in my magnetically permeable cast iron composition, smallamounts of other alloying elements may be added in order to more readilyattain the desired microstructure of ferrite and temper carbon ornodular graphite. Such a microstructure in cast irons is commonlyproduced in the prior art in ferritic malleable iron and ferriticnodular iron. Malleable iron of course, is produced from a white castiron by a suitable annealing cycle. Nodular iron on the other hand isproduced from the molten alloy by the addition of inoculants, such asmagnesium and cerium, which cause the uncombined carbon to separate asnodules or spherulites of graphite. Depending upon the base compositionof the alloy, the amount of the inoculants, and the rate of cooling asubsequent anneal, may be required to convert residual pearlite toferrite. It is known that minor amounts of alloying elements may beadded to aid in the formation of a suitable ferritic malleable orferritic nodular iron and they will be discussed below in more detail.While these alloying elements do not directly improve the magneticpermeability of ferrous alloys, it is apparent that they contribute tothe practice of my invention by enhancing formation of graphitic tempercarbon or nodules. A ferritic malleable iron of the above-definedcomposition with respect to carbon, silicon, and titanium is thepreferred form of my cast magnetically permeable alloy.

In the production of ferritic malleable iron, it is desirable thatcarbon remain in combined form until after the molten alloy has beencast and has solidified as white cast iron. To this end it is known thatbismuth and/ or tellurium may be added as mild carbide stabilizers toprevent mottle during casting. Bismuth and/ or telluriurn mayeffectively be 'added for this purpose in quantities from about 0.002-0.2% by weight. To balance the carbide stabilizing effect of the bismuthand tellurium, it is also known that small amounts of boron mayeffectively be added. The boron does not materially affect iron carbidesduring casting but it does accelerate carbide decomposition duringsubsequent annealing. Boron is preferably addedin an amount equivalentto about 00001-0002 percent by weight of the alloy. Tellurium may beadded as ferrotellurium or as tellurium metal. Bismuth may be added asbismuth metal or as bismnith alloyed with boron. Boron may beeffectively added as 'ferroboron, as an alloy with bismuth, or as aborate as for example sodium borate.

It-is also known that the spherulitic graphite structure character ofnodular iron is produced by the addition of one or more suitableelements to the molten cast iron. For example, magnesium, cerium,columbium, lithium, sodium, barium and other elements will produce thesphenulitic graphite structure. of these elements magnesium and ceriumare commercially important and in many applications both magnesium andcerium are employed. Suflicient magnesium is added to obtain a residualamount of magnesium usually not in excess of 0.08%. As is well known inthe art, a nodular iron microstructure containing essentially pearliteor essentially ferrite or mixtures thereof may be obtained as desired.

The production of both malleable iron and nodular iron is well known tometallurgists and either method may be used to produce my compositionprovided the final cast alloy is essentially free of matrix pearlite andof free graphite. A small amount of rim pearlite can be tolerated but isnot preferred. Carbon either in the form of pearlite or of flakegraphite adversely affects the magnetic permeability of the cast ferrousalloy particularly in the relatively unsaturated portion of thehysteresis curve.

As is known, it is very difficult to eliminate sulfur which is almostalways present in small amounts in cast irons. Since sulfur stabilizescementite and thus pearlite, it is preferable that its carbidestabilizing effect be neutralized by incorporating a small but effectiveamount of manganese. In accordance with my invention, it is preferredthat the sulfur content be kept below a maximum of about 0.1 5- 020%sulfur. Accordingly, it is preferred to add about 0.30-0.60% manganeseas for example ferromanganese which will assure a proper balance betweenthe manganese and sulfur contents.

An example of a preferred embodiment will further describe the manner inwhich my invention may the practiced. An alternator rotor pole was castsuitable for use in automobile alternators such as is found on presentmodel automobiles. The composition of the cast alloy comprised byweight, 2.34% carbon, 1.8% silicon, 0.37% manganese, 0.134% sulfur,0.10% titanium and the balance substantially all iron. The melt wasprepared in an induction furnace using scrap iron as the base metal.Titanium was added as 30% ferrotitanium and silicon was added as siliconmetal. These additions were made to the melt in the induction furnace atapproximately 2750 F. The tapping temperature was 2800 F. The rotor wascast in a green sand mold with an oil sand core. After the casting hassolidified and cooled, it was removed from the sand mold and cleaned. Itwas then subjected to the following annealing cycle to produce anessentially ferritic malleable iron. The casting was heated in asuitable furnace over a period of three hours to 1550 F. In the nextthree and three quarter hours the temperature was slowly increased to1725 F. at which temperature the casting was then maintained for aperiod of about eight and a half hours. The cast rotor was then cooledfrom 1725 F. to 1375 F. in about another one and a half hours. Thecasting was maintained at a temperature between about 1325 F. and 1375F. for a period of an additional eight hours at which time the castingwas air cooled to room temperature. Thus approximately twenty-four hoursare required for the complete malleabilizing heat treatment. At thecompletion of the heat treatment the microstructure of the castingconsisted only of ferrite and graphitic temper carbon. No comlbinedcarbon or flake graphite was apparent.

The cast rotor pole was then incorporated into an otherwise standardcommercially available alternator and the following magnetic propertiesof the component were determined:

Flux density Magnetic force oersteds: gausses 8,800

From the tabulated data it is observed that the magnetic permeability ofthe above specimen is 1760 gausses per oersted at 5 oersteds and about218 gausses per oersted at 68 oersteds. Other cast iron composiitonswithin the limits specified have all been found to have magneticpermeability values of, or in excess of, 1600 or 210 gausses per oerstedwhen subjected to magnetic forces of .5 or 70 oersteds respectively. Itwill be apparent to one skilled in the art that an alternator rotor poleof my ferrous composition is entirely suitable for use in automotivealternators. Moreover, the pole may be manufactured by a casting processwhich is preferred to low-carbon steel stampings currently in use.

While my invention has been described in terms of a specific embodiment,it appears that other applications can be devised by those skilled inthe art and therefore the scope of my invention is intended to belimited only by the following claims.

I claim:

1. A ferritic cast iron of high magnetic permeability having a matrixconsisting essentially of ferrite and being free of flake graphite, saidalloy consisting essentially by weight of about 0.010.10% titanium,2.22.7% carbon, 1.22.0% silicon and the balance 1r0n.

2. A ferrous cast alloy of high magnetic permeability having a matrixconsisting essentially of ferrite and being free of flake graphite, saidalloy consisting essentially by Weight of about 0.01-0.l0% titanium,2.22.7% carbon, 1.2-2.0% silicon, 0.30-0.60% manganese, sulfur not inexcess of 0.15%, phosphorus not in excess of 0.05% and the balance iron.

3. A cast ferritie malleable iron characterized by a magneticpermeability in excess of 1600 gausses per oersted at a magnetic forceof about 5 oersteds and 210 gausses per oersted at a magnetic force ofabout 70 oersteds, said ferritic malleable iron consisting essentiallyby weight of about 0.010.10% titanium, 2.2-2.7% carbon, 1.22.0% silicon,0.30-0.60% manganese, 0.0020.02% of at least one element selected fromthe group consisting of bismuth and tellurium, 0.00010.002% boron, andthe balance iron, said alloy being characterized by a microstructurewhich is essentially free of flake graphite and matrix pearlite andcontains only a minor amount of rim pearlite.

References Cited UNITED STATES PATENTS 1,636,657 7/1927 Schwartz 148-138X 1,707,753 4/1929 Boegehold 148-35 X 2,501,059 3/1950 Kluijtmans 14835X 2,579,452 12/1951 Eckman et al. -123 2,901,386 8/1959 Saives 14835 X3,189,492 6/1965 Laudenslager et al. 75123 X FOREIGN PATENTS 240,0176/1960 Australia.

CHARLES N. LOVELL, Primary Examiner.

US. Cl. X.R.

